Method of pulse control of high-voltage rectifiers, for example thyristors, operating in a polyphase converter, and a control system to realize said method

ABSTRACT

A method and system are characterized in that narrow control pulses are applied to the rectifiers in the presence of a positive voltage across them at the beginning of their conduction interval and at the subsequent passages of the voltage across each rectifier through zero from a negative to a positive value during the entire conduction interval of the rectifier. The method keeps to a minimum overvoltages in converters operating in the 0-1 mode and completely eliminates the presence of a positive signal at the control electrodes of the thyristors dangerous to them.

United States Patent n 1 Travin et al.

[ 1 3,715,606 51 =Feb.6, 1973 IN A POLYPHASE CONVERTER,AND A CONTROLSYSTEM TO REALIZE SAID METHOD [76] Inventors: Lev Viktorovich Travin,Leninsky prospekt, 101, korpus 136, kv. 125; Valentin Georgievich Spuv,ulitsa Tukhachevskogo, 28, korpus l, kv. 49, both of Moscow, U.S.S.R.

[22] Filed: Dec. 24, 1970 [21] Appl. No.: 101,348

[52] US. Cl. .....307/252 Q, 307/252 UA, 323/22 SC [51] Int. Cl. ..H03k17/00 [58] Field of Search ..307/252 0, 252 UA, 252 N, 252 T, 307/252 W;323/22 SC [56] References Cited UNITED STATES PATENTS 3,283,179 11/1966Carlisle et al. ..30 7/252 T Sen Htgfi-va/iage rectifier /valvy/Transmitter 5mm AN/i distribular amplz/Zer gait l0/ l 966 4/ l 969 Spink..307/252 0 Jednacz ..307/252 0 Primary Examiner-John ZazworskyAttorney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT 5 Claims,SDrawing Figures METHOD OF PULSE CONTROL OF HIGH- VOLTAGE RECTIFIERS,FOR EXAMPLE TIIYRISTORS, OPERATING IN A POLYPHASE CONVERTER, AND ACONTROL SYSTEM TO REALIZE SAID METHOD The present invention relates tothe control of highvoltage converters, and more specifically to a methodfor the pulse control of high-voltage rectifiers such as, for example,thyristor valves or high-voltage mercuryarc valves operating in apolyphase converter, and to a system for the pulse control of suchrectifiers.

High-voltage thyristor valves or high-voltage mercury-arc valves areessential elements of any high-voltage polyphase converter and are usedas the arms of the converter. A high-voltage thyristor valve alsodesignated herein a high-voltage thyristor unit usually consists of alarge number of thyristors connected in series -parallel together withall necessary auxiliary circuits for voltage and current distributionbetween thyristors, protection of thyristors, and so forth. I

The requirements that systems for the pulse-control of high-voltagerectifier units should meet may be divided into two groups. Therequirements in the first group stem from the fact that a large numberof thyristors connected in series-parallel have to be switched onsimultaneously and consist essentially in that control pulses shouldhave a sufficiently large amplitude, a sufficiently high rise time, anda sufficiently long duration, and that there should be the leastpossible difference in waveform and in time between the pulses appliedto the thyristors of the same unit. The fulfilment of these requirementsserves to maintain a uniform distribution of the voltage between theseriesconnected thyristors at the instant when the high-voltagethyristor unit is turned on.

The requirements in the second group are pivoted to the mode ofoperation of and the transients in a par ticular converter such as, forexample, a twelve-phase cascoded-bridge converter which is made up oftwo series-connected three-phase bridge circuits and is widely used ind.c. power transmission lines. In such a case, the principal mode ofoperation determining the characteristics of the control pulses is suchthat the rectifiers (units) in the bridge circuit are conducting in turnand one at a time which is known as the 0-1 mode. This mode may occur ina number of transients accompanied by a decrease in the rectifiedcurrent in the associated d.c. power transmission line, such as when thed.c. power transmission line is turned on or off, or automaticallyreclosed, or when bridge is being shorted out or brought in to circuit,or when it is being changed over to inverter operation, and the like.The 0-1 mode is characterized by the fact that at the instant whencontrol pulses are applied to the rectifiers of the anode and cathodegroups, the voltages across them may be of opposite polarities.

In such a case, if the duration of the control pulses is short, onlythat rectifier will fire which accepts a positive voltage. Thisrectifier will pass the re-charging current of the circuit capacitance(for example, that of a converter transformer) to ground and willextinguish after this current has passed through zero. When narrowcontrol pulses are applied to a next pair of rectifiers, a rectifier inthe othergroup will fire, and the 0 steady-state mode of operation andthe direct current of the converter is continuous. A normal conductioninterval of a rectifier depends on the circuit of the converter where itis used; e.g., the normal conduction interval of any rectifier (arm) ina three-phase bridge converter is equal to one-third of the period ofthe ac. voltage supplied to the converter plus the overlap angle.

As the circuit capacitances are re-charged, the voltage across the otherdead rectifier grows positive and, if a control pulse is applied to itat that instant, it will fire, thereby upsetting the 0-1 mode andlimiting overvoltages.

An analysis of operation of mercury-arc rectifiers in.

the converters of d.c. power transmission lines has shown that the bestmethod to limit overvoltages in the 0-1 mode of operation is to use acontrol pulse the duration of which exceeds the re-charge time when onerectifier fires. As applied to the control of mercury-arc rectifiers,this means that each rectifier should be driven with a single broadcontrol pulse electrical degrees wide (see, for example, U.S. Pat. No.3218541, Class 321-48, 1965, POLYPHASE ELECTRICAL CONVERTER).

Broad control pulses with a duration equal to the conduction interval ofthe rectifier fully eliminate the occurrence of the 0-1 mode, but hismethod of control suffers from a number of disadvantages when it is usedto control high-voltage thyristor units:

in most existing systems for control'of high-voltage thyristor units thecircuits responsible for the distribution of control pulses between thethyristors of a unit cannot transmit control pulses of such a duration,while in other control systems this requirement entails complications inthe circuit configurations;

with such a duration of control pulses, it is difficult to secure apulse rise time of 0.2 3;]. sec, especially in view of the necessity toinsure the requisite insulation level of the control circuits.

As regards a high-voltage thyristor unit, this pulse rise time isnecessary so that the applied voltage can be uniformly divided among thehundreds of series-connected thyristors, because in this case thedifference in time of firing between the individual thyristors isreduced to a minimum.

In order to eliminate these disadvantages, the single broad pulse isknown to have been replaced with a series of short control pulses suchthat the duration of the pulse series is equal to the conductioninterval of the rectifier (see, for example, U.S. Pat. No. 3315098,Class 307-885, 1967, GATING CIRCUIT FOR CONTROLLED RECTIFIER").

A series of narrow control pulses eliminates the above mentioneddisadvantages but the overvoltages occurring in the converter in the 0-1mode may still be considerable and will depend on the duration of andthe spacing between the narrow control pulses.

Besides, both of the above mentioned methods suffer from otherdisadvantages:

1. With a single broad pulse or a series of narrow pulses used tocontrol a high-voltage thyristor unit, and with the converter operatingon an intermittent current, a case is inevitable when in the presence ofa positive control pulse the voltage across the high-voltage thyristorunit is negative. In such a case, the reverse currentsof the thyristorsgrow tens or even hundreds of times, which can increase the powerdissipated by the thyristors to a point where a thermal break-down canoccur, and alsoa non-uniform division of the inverse voltage among thehundreds of series-connected thyristors of the unit and, as a result,the electrical break-down of some of them. Because of this, broad pulsesor a series of narrow pulses may prove unsuitable for control of ahigh-voltage thyristor unit.

2. Circuits shaping and amplifying broad control pulses or a series ofnarrow control pulses draw much power, and this entails an increase inthe power dissipated by the mercury-arc rectifier or high-voltagethyristor unit proper and stands in the way of utilizing a verypromising method by which a mercury-arc rectifier orhigh-voltagethyristor unit draws its feed voltage directly from the applied voltage.

An object of the present invention is to eliminate the above mentioneddisadvantages.

A more specific object of the invention is to provide a method for thepulse control of high-voltage rectifiers used in polyphase convertersand a control system based on the method disclosed herein, whichminimize overvoltages in conversion circuits operating in the -1 mode,completely eliminate the appearance of the dangerous positive signal atthe control electrodes (gates) of the thyristors in the unit when thereis a negative voltage across that unit, and simplify the problem offeeding the circuit shaping and amplifying the control pulses directlywith the voltage applied to the thyristor unit.

These objects are accomplished by the fact that in the method disclosedherein for the pulse control of high-voltage rectifiers used inpolyphase converters, based on the application of narrow control pulsesto the rectifiers (i.e., to the gates of the thyristors of high-voltagethyristor valves or to the grids of mercury-arc valves), the said narrowcontrol pulses are, according to the invention, applied to therectifiers in the presence of a positive voltage at them, at thebeginning of their conduction interval and at the subsequent instantswhen the voltage across .each rectifier passes through zero from anegative to a positive value during the entire conduction interval ofthe rectifier.

in a system for the pulse control of said high-voltage rectifiers,comprising a phase-inverter to change the phase of control pulses, asynchronizer to furnish pulses which determine the onset and duration ofthe conduction intervals of the rectifiers, a control-pulseshaperamplifier, and a device to distribute said control pulses, thereis, according to the invention, for each rectifier a feedback circuitcomprising a device sensing the polarity and passage of the rectifiervoltage (the voltage across said rectifier or valve) through zero, aninformation link for linking the system elements having differentpotentials, and an AND gate one input of which is connected to theoutput of the synchronizer, the other input of which is connected tothe. output of the sensor of the polarity and passage of the rectifiervoltage through zero, and the output of the AND gate is connected to theinput of the control-pulse shaper-amplifier.

This arrangement enables narrow control pulses to be applied to therectifiers in the presence of a positive voltage across them, at thebeginning of their conduction interval and at the subsequent instantswhen the voltage across each rectifier passes through zero from anegative to a positive value during the entire conduc tion interval ofthe rectifier.

To transmit information about the instants when the rectifier voltagepasses through zero from a negative to a positive value during theconduction interval of the rectifier, it is preferable to arrange theinformation link betweenthe sensor of the polarity and passage of therectifier voltage through zero and the second input of the AND gate.

To transmit information about the onset and duration of the conductionintervals of the rectifier, the information link may be arranged betweenthe synchronizer and the first input of the AND gate.

In case where use is made of a source of narrow synchronizing pulses,between the output of the synchronizer and the first input of the ANDgate there should preferably be a pulse circuit to transform the narrowsynchronizing pulsesinto broad pulses with a duration equal to theconduction interval of the rectifier. The pulse circuit may be aone-shot multivibrator.

The method and system for pulse control of highvoltage rectifiersdisclosed herein offer a number of advantages over their control bymeans of broad control pulses or a series of narrow control pulses,namely:

the method disclosed herein is applicable to any system of distributionof control pulses among the thyristors of a unit, especially as regardsthe promising light-control systems;

the method keepsto a minimum overvoltages in the converter operating inthe 0-1 mode, while in cases where use is made of a series of narrowcontrol pulses the magnitude of overvoltages in the 0-] mode depends onthe duration of and, the spacing between the narrow control pulses;

the method completely eliminates a mode of operation dangerous to thethyristors of they unit, namely, the presence of a positive signal atthe control electrodes (gates) of the thyristors of the unit when thereis a negative voltage across the thyristor unit;

in most systems for control of high-voltage thyristor ses, it draws muchless power than circuits shaping and amplifying broad control pulses ora series of narrow control pulses. As a consequence, the method forcontrol of high-voltage rectifiers disclosed herein simplifies theproblem of feeding the control-pulse shaper-amplifier directly with thevoltage across the thyristor unit.

It should be noted that the method and system disclosed herein are alsoapplicable to the control of mercury-arc rectifiers.

The invention will be more fully understood from the following moredetailed description of a method for pulse control of high-voltagerectifiers and a system for control of these rectifiers, when read inconnection with the accompanying drawings wherein:

FIG. 1 is a block-diagram of an embodiment of the invention in the formof a system for the control of one converter rectifier, in which aninformation link is arranged between a device sensing the polarity andpassage of rectifier voltage through zero and the second input of ANDgate;

FIG. 2 is a block-diagram of a modification of the system of FIG. 1, inwhich the information link is arranged between a synchronizer and thefirst input of the AND gate;

FIGS. 3 and 4 are voltage waveforms illustrating operation of the systemdisclosed herein; and

FIG. 5 is a schematic diagram of a second embodiment of the invention inthe form of a system for the pulse control of a converter rectifier.

The method and a system for its realization as disclosed herein willnext be considered as applied to the pulse control of one high-voltagerectifier operating in a polyphase converter.

In each of FIGS. 1 and 2, there is a system for the control of aconverter rectifier, said system comprising the following keycomponents: a phase-inverter l to change the phase of control pulseswithin the requisite limits determined by the regulating signal uapplied to its input; a synchronizer 2 to furnish pulses which determinethe onset and duration of the conduction interval of the rectifier; ashaper-amplifier 3 to shape and amplify control pulses; and a device 4to transmit and distribute control pulses to the control electrodes ofthe rectifier 5.

Additionally, the control system has, according to the invention, afeedback circuit comprising a device 6 sensing the polarity and passageof the voltage across the rectifier 5 through zero from a negative to apositive value, an information link 7 to link the circuit componentshaving different potentials, and an AND gate 8.

The control system with said feedback circuit has some of its componentsat ground potential and some of them at'the high-voltage potential of arectifier (or its platform). The information link 7 in FIGS. 1 and 2 isprovided not only to link the corresponding system components havingdifferent potentials, but also to provide the necessary isolationbetween them, as will be shown hereinbelow.

The system in FIG. 1 just described operates as follows:

The signal carrying information about the requisite firing angle (theinstant of the beginning of the conduction interval of the rectifier 5)goes from the phase-inverter 1 to the input of the synchronizer 2 at theoutput of which there appears a broad rectangular pulse. The leadingedge of this pulse determines the requisite firing angle, and itsduration is equal to the specified conduction interval of the rectifier5. The conduction interval may be constant and decided solely by thechosen configuration of the converter circuit, or it may be a variableone (during transients) and decided by both the converter circuit andthe intervals between the instants at which the switching rectifiers (inone group of the converter rectifiers) fire in the process of regulatingthe firing angles.

The broad pulse appearing across the output of the synchronizer 2 goesto one of the inputs of the AND gate 8. The other input of the AND gateis fed via the information link 7 (which may be a pulse isolatingtransformer, a light link, a radio link, and the like) with a sequenceof pulses from the sensor 6 connected in parallel with the rectifier 5,which represent the polarity and passage of the rectifier voltage 5through zero. As a result of the subsequent differentiation, the outputof the AND gate delivers narrow pulses at the beginning of thesynchronizing pulse (if the voltage across the rectifier is positivethen) and also each time the rectifier voltage passes through zero froma negative to a positive value during the conduction interval equal tothe duration of the synchronizing pulse. These pulses go to the input ofthe shaper-amplifier 3 and, after being properly shaped and amplified,the narrow control pulses are routed by the control-pulse distributor 4to the control electrode of the high-voltage rectifier 5.

The number of pulses at the output of the AND gate and, as aconsequence, the number of control pulses applied to the rectifier 5during one conduction interval is automatically decided by the number ofpassages of the rectifier voltage through zero during the conductioninterval from a negative to a positive value. In most cases, when aconverter is operating under steady-state conditions and with a directcurrent flowing continuously, a positive voltage is applied across eachrectifier of a converter during its entire conduction interval; i.e.,this voltage does not change sign during the conduction interval, and,each rectifier is driven by only one narrow control pulse at thebeginning of the conduction interval. In the control system disclosedherein, control pulses cannot be applied to a rectifier outside theconduction interval or when there is a negative voltage across therectifier at any instant during the conduction interval.

The block-diagram of the control system of FIG. 1 applies to a case whenthe key elements of the control system, namely the phase-inverter 1, thesynchronizer 2, the AND gate 8, and the control-pulse shaper-amplifier 3are at ground potential, and the sensor 6 is at the platform potentialof the rectifier 5. The requisite isolation of the control-systemelements held at different potentials is insured by the control-pulsedistributor 4 and the information link 7. The system representedby theblock-diagram of FIG. 1 is particularly convenient when the thyristorsof a high-voltage thyristor unit are light-radio controlled.

In cases where the control of the rectifiers 5 utilizes a transformer, acable-transformer, or any other electromagnetic system for thetransmission and distribution of control pulses, it is preferable toplace the above listed elements 3, 4, 6 and 8 of the control system atthe rectifier platform, and to transmit a synchronizing pulse with aduration equal to the conduction interval from the synchronizer 2 (heldat ground potential) to the AND gate (held at the potential of therectifier platform) via the information link 7. The block-diagram ofsuch a control system is shown in FIG. 2. The system operates in muchthe same way as that shown in FIG. 1, with the only exception that inthis case the information link 7 transmits from the synchronizer 2 togate 8 synchronizing pulses and not information about the voltage acrossthe rectifier 5, as is the case in the system represented by theblock-diagram of FIG. 1.

The voltage waveforms shown in FIGS.3 and 4 illustrate operation of thesystem in a case when at the beginning of the, conduction interval, apositive volt? age is applied to the rectifier (FIG. 3) and when thevoltage across the rectifier 5 at the same instant t is negative(FIG.4).

For simplicity, it is assumed that:during the conduction interval t, tthe rectifier 5 goes out only once (at time that a negative voltage isapplied to it during the interval t, t and that at time the voltageacross the rectifier 5 again becomes positive. The following notation isadopted in FIGS.3 and 4: 14 is the voltage across the output of thesensor 6, applied to the first input of the AND gate and representingthe specified conduction interval of the rectifier 5; u is the voltageacross the output of the synchronizer, applied to the second input ofthe AND gate; u represents the narrow pulses (after differentiation)across the output of the AND gate; u; represents the control pulsesappearing across the output of the shaper-amplifier 3 and applied to therectifier via the pulse distributor 4.

In the first case FIG.3), the control pulses are applied to therectifier 5 at time 1, corresponding to the onset of the conductioninterval (at this instant, the polarity of u,, correspondsto a positivevoltage across the rectifier 5) and at time corresponding to the passageof the rectifier voltage through zero from a negative to a positivevalue. I

In the second case (FIG. 4), the first control pulse is applied to therectifier 5 at time t,, that is, at the instant when the rectifiervoltage passes through zero for the first time (since at time afterthebeginning of the conduction interval of the rectifier 5 at t t, thevoltage across the rectifier 5 is negative), and the second controlpulse is applied at time t,, corresponding to the second passage of thevoltage across the rectifier 5 through zero. At any other instants, nocontrol pulses are formed or applied to the rectifier S.

In some cases, the synchronizer is arranged so that instead of a broadcontrol pulse it delivers either one narrow pulse determining the onsetof the conduction interval or two narrow pulses determining the onsetand termination of the conduction interval. For example, in the systemrepresented by the block-diagram of FIG.2, instead of a broadsynchronizing pulse, it may sometimes be preferable to transmit from thesynchronizer 2 and over the link 7 either one narrow pulse determiningthe onset of the conduction interval or two narrow pulses determiningthe onset and termination of the conduction interval. In all such cases,it is preferable. to place an additional pulse circuit 9 (shown by thedashed blocks in FIGS. 1 and 2 between the synchronizer 2 and the ANDgate to convert the narrow synchronizing pulses into broad synchronizingpulses the duration of which is equal to the specified conductioninterval of the rectifier. This pulse. circuit may be a flip-floptriggered by two narrow pulses from the synchronizer 2 (in the case of avariable conduction interval) or a one-shot multivibrator triggered byone narrow synchronizing pulse (in the case of a fixed conductioninterval).

FIG.5 is the schematic diagram of the elements 6, 8 and 9 of the systemdisclosed herein according to FIG.2, where the said elements 6, 8, and 9are held at the platform potential of the rectifier 5.

The rectifier 5 is shown in FIG. 5 is a high-voltage thyristor valve,consisting of a large number of thyristors 34 connected in series; theauxiliary circuits of the high-voltage thyristor valve are not shown-inFIG. 5. The valve has an anode terminal 24, a cathode terminal 25 andcontrol terminals 26, connected with the control electrodes (gates) ofthe thyristors of the valve. The anode and cathode terminals 24, 25 ofthe high-voltage thyristor valve are connected to the correspondingpoints of a converter (not shown in FIG. 5). The platform (base) of thevalve has the potential of its cathode, which is a high-voltagepotential relative to ground.

Phase-inverter l and synchronizer 2 are of any usual type producing anarrow synchronizing pulse at the beginning of each conduction intervalof the high-voltage thyristor valve 5 and are placed at groundpotential. The information link 7 which is of a known type and may be apulse-isolating transformer or a system consisting of a light-pulsesource, an optical channel and a light-pulse receiver provides the linkbetween the synchronizer 2 and the one-shot multivibrator-amplifier 9and the required isolation between them. The input terminals 27, 28 ofthe element 9 are connected to the output of the information link 7.

The sensor of the polarity and passage of the rectifier voltage (thevoltage across the high-voltage thyristor valve 5) through zeroincorporates a resistor 29 of high resistance a symmetrical Zener-diodevoltage limiter built around Zener diodes l0 and 11, a voltage dividerl2 and a two-stage amplifier based on transistors 13 and 14.

The output signal of the sensor 6 (the voltage developed across aresistor R is applied to the first input (a diode l5) ofthe AND gate 8built around the transistor 16. The narrow synchronizing pulse comingfrom the synchronizer 2 over the information link 7 is applied to theinput 27 28 of the one-shot multivibrator 9 based on transistors 17 and18, which delivers a broad pulse the duration of which is equal to theconduction interval of the rectifier 5, decided by the configuration ofthe converter circuit. After amplification by a three-stage amplifierusing transistors 19, 20 and 21,.this broad pulse is applied to thesecond input (a diode 22) of the AND gate. The signals at the output ofthe AND gate (the voltage across a resistor R,) will appear only whenthe respective signals are applied simultaneously to both inputs of theAND gate. The narrow pulses produced as a result of differentiation by aC R network are amplified by a pre-amplifier built around a transistor23 and are then applied via output terminals 30 and 31 or thecoincidence gate 8 to the controlpulse shaper-amplifier 3.

The control-pulse shaper-amplifier 3 forms and amplifies control pulseswith specified amplitude, duration and rate-of-rise, which are appliedto the control electrodes (gates) 26 of the high-voltage thyristor valvevia a control pulse distributor 4. The shaper-amplifier 3 and thecontrol-pulse distributor 4 are of known kinds and may be either a powerpulse generator with a set of isolating pulse transformers, or a sourceof power light pulses with a set of optic fiberglass guides and with aset of circuits (one such circuit for one individual thyristor or forseveral thyristors) transforming light pulses into electrical controlpulses of required amplitude, duration and rate-of rise. Otherconfigurations and constructions of elements 3 and 4 are also possible.

The power supply of elements 6, 8 and 9 of the control system in FIG. 5(and elements 3 and 7 if necessary) may be accomplished by any knownsupply means via the supply terminals 31, 32 and 33 in FIG. 5, e.g., bya battery, by an isolating supply transformer and rectifier (from anauxiliary a.c. network of a converter) or more preferably by a rectifiersupplied from the voltage across the high-voltage thyristor valve viagrading or damping circuits of this valve.

The same configurations of sensor 6, coincidence gate 8 and one-shotmultivibrator 9 as shown in FIG. 5 may be used in the control systemshown in FIG. 1. The rectifier 5 in FIGS. 1 and 2 may be a high-voltagemercury-arc valve instead of a high-voltage thyristor valve.

What is claimed is:

1. A system for pulse control of high-voltage rectifiers includingcontrol electrodes and having conduction intervals and operating in apolyphase converter, comprising a plurality of system components adoptedto operate at different potentials and including a phase-inverter tovary the phase of control pulsesand including an output; a synchronizerincluding an output and connected to the output of said phase-inverterand determining the onset and duration of conduction intervals of therectifiers; a control-pulse shaper-amplifier including an output and aninput; a control-pulse distributor connected to the output of saidshaper-amplifier to transmit and distribute control pulses to thecontrol electrodes of the rectifier; a feedback circuit comprising asensor including an output and sensing the polarity and passage of therectifier voltage through zero and connected in parallel with therectifier, an in-,

formation link for linking the system components operating at differentpotentials, and an AND gate including two inputs one of which isconnected to the output of said synchronizer and the other of which isconnected to the output of said sensor of the polarity and passage ofthe rectifier voltage through zero, said gate including an outputconnected to the input of said control-pulse shaper-amplifier, whilesaid feedback circuit enables narrow'control pulses to be applied to therectifier in the presence of a positive voltage across it at thebeginning of its conduction interval and also at the subsequent instantswhen the rectifier voltage passes through zero from a negative to apositive value during the entire conduction interval of the rectifier.

2. A system, as claimed in claim 1, in which to transmit informationabout the instants when the rectifier voltage passes through zero from anegative to a positive value during the'conduction interval of therectifier the information link is arranged between the sensor of thepassage of the rectifier voltage through zero and the second input ofthe AND ate.

3. A system, as claime in claim 1, in which to transmit informationabout the onset and duration of conduction intervals of the rectifierthe information link is arranged between the synchronizer and the firstinput of the AND gate.

4. A system, as claimed in claim 1, comprising a pulse circuit and inwhich, when use is made of a source of narrow synchronizing pulses, saidpulse circuit, which is coupled between the output of the synchronizerand the first input of the AND gate, transforms the narrow pulses of thesynchronizer into broad pulses the duration of which is equal to theconduction interval of the rectifier.

5. A system, as claimed in claim 4, in which said pulse circuit is aone-shot multivibrator.

1. A system for pulse control of high-voltage rectifiers includingcontrol electrodes and having conduction intervals and operating in apolyphase converter, comprising a plurality of system components adoptedto operate at different potentials and including a phase-inverter tovary the phase of control pulses and including an output; a synchronizerincluding an output and connected to the output of said phase-inverterand determining the onset and duration of conduction intervals of therectifiers; a control-pulse shaper-amplifier including an output and aninput; a control-pulse distributor connected to the output of saidshaper-amplifier to transmit and distribute control pulses to thecontrol electrodes of the rectifier; a feedback circuit comprising asensor including an output and sensing the polarity and passage of therectifier voltage through zero and connected in parallel with therectifier, an information link for linking the system componentsoperating at different potentials, and an AND gate including two inputsone of which is connected to the output of said synchronizer and theother of which is connected to the output of said sensor of the polarityand passage of the rectifier voltage through zero, said gate includingan output connected to the input of said control-pulse shaper-amplifier,while said feedback circuit enables narrow control pulses to be appliedto the rectifier in the presence of a positive voltage across it at thebeginning of its conduction interval and also at the subsequent instantswhen the rectifier voltage passes through zero from a negative to apositive value during the entire conduction interval of therectifier.
 1. A system for pulse control of high-voltage rectifiersincluding control electrodes and having conduction intervals andoperating in a polyphase converter, comprising a plurality of systemcomponents adopted to operate at different potentials and including aphase-inverter to vary the phase of control pulses and including anoutput; a synchronizer including an output and connected to the outputof said phase-inverter and determining the onset and duration ofconduction intervals of the rectifiers; a control-pulse shaper-amplifierincluding an output and an input; a control-pulse distributor connectedto the output of said shaper-amplifier to transmit and distributecontrol pulses to the control electrodes of the rectifier; a feedbackcircuit comprising a sensor including an output and sensing the polarityand passage of the rectifier voltage through zero and connected inparallel with the rectifier, an information link for linking the systemcomponents operating at different potentials, and an AND gate includingtwo inputs one of which is connected to the output of said synchronizerand the other of which is connected to the output of said sensor of thepolarity and passage of the rectifier voltage through zero, said gateincluding an output connected to the input of said control-pulseshaper-amplifier, while said feedback circuit enables narrow controlpulses to be applied to the rectifier in the presence of a positivevoltage across it at the beginning of its conduction interval and alsoat the subsequent instants when the rectifier voltage passes throughzero from a negative to a positive value during the entire conductioninterval of the rectifier.
 2. A system, as claimed in claim 1, in whichto transmit information about the instants when the rectifier voltagepasses through zero from a negative to a positive value during theconduction interval of the rectifier the information link is arrangedbetween the sensor of the passage of the rectifier voltage through zeroand the second input of the AND gate.
 3. A system, as claimed in claim1, in which to transmit information about the onset and duration ofconduction intervals of the rectifier the information link is arrangedbetween the Synchronizer and the first input of the AND gate.
 4. Asystem, as claimed in claim 1, comprising a pulse circuit and in which,when use is made of a source of narrow synchronizing pulses, said pulsecircuit, which is coupled between the output of the synchronizer and thefirst input of the AND gate, transforms the narrow pulses of thesynchronizer into broad pulses the duration of which is equal to theconduction interval of the rectifier.